In this presentation, we will discuss the emission mechanism and ultrafast electron dynamics of laser-induced electron emission from a tungsten tip for weak and strong optical fields based on measured electron energy spectra and simulations.
Illuminating a sharp metallic tip with femtosecond laser pulses produces spatially and temporally confined electron pulses by plasmonic effects at the tip apex , and created coherent pulsed field emission with spatio-temporal control with femtosecond and nanometer resolution [2, 3]. The emission mechanism depends on the strength of the laser field. For relatively weak fields, single-electron excitations by single- and multi-photon absorption are prevalent, and photo-excited electrons are either tunneling through the surface potential barrier or being emitted above the barrier . On the other hand, very strong fields largely modify the surface barrier and drive direct tunneling emission from the Fermi energy through the barrier, what is termed optical field emission [5, 6].
Here, we have investigated electron energy spectra of the electron emission induced by 7 fs laser pulses from a clean tungsten tip apex. By measuring energy spectra with varying laser power, a smooth transition of emission mechanisms from the weak-field regime to the strong-field regime was observed. In strong laser fields, we confirmed the appearance of optical field emission. It is characterized by the opening of a peculiar emission channel. This channel involves prompt laser-driven tunneling emission and subsequent laser-driven electron re-scattering off the surface, delayed by the electrons traveling far inside the metal before scattering .
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